Method for preparing a paper product



United States Patent 2,786,757 METHOD FOR PREPARING A PAPER PRODUCTLouis B. Taylor, Pittsburgh, Pa., assignor to Columbia- SouthernChemical Corp., Allegheny County, Pa., a corporation of Delaware NoDrawing. Continuation of application Serial No. 352,295, April 30, 1953.This application June 28, 1956, Serial No. 594,390

9 Claims. (Cl. 92-21) This invention relates to a novel method forpreparing a paper product.

Paper made according to this invention has high brightness and opacity,and is suitable for use as writing paper, or as top liner for boxboardcartons, or for other purposes where paper having high brightness andopacity is required.

According to this invention, the herein contemplated paper may beprepared by forming a paper pulp dispersion in an aqueous acidicmaterial which forms a substantially water-insoluble (less than about 5or 6 grams per liter) salt of an alkaline earth metal and adding calciumsilicate or equivalent alkaline earth metal silicate to the acidicslurry.

Any type of paper pulp may be used in making paper according to thisinvention. Chemically-treated pulps such as sulfite and kraft may betreated according to this invention, which is also applicable to groundwood and semi-chemical pulps. Either bleached or unbleached pulps may beused. Rags may also be included in the pulp if desired. Broke or scrapsand cuttings of finished paper may be used in addition to pulp, and theuse of broke reduces the pulp requirements. Paper wasted inmanufacturing operations is generally available and may be used asbroke.

Brightness and opacity of paper made from any type of pulp are improvedby practice of the present invention, but the improvement is most markedin paper made from low-grade pulps. Paper made according to the presentinvention compares favorably in brightness, opacity and strength topaper pigmented with titanium dioxide, and may be produced at less cost.

The process of this invention may be performed by mixing the acid oracidic material with the pulp with or without beating and then addingthe alkaline earth metal silicate to the acidified slurry. An especiallygood product is obtained when sulphuric acid, sulphurous acid, oraluminum sulphate is used as the acidic material, and calcium silicateis the alkaline earth metal silicate. However, other acidic substancesand other alkaline earth metal silicates may be used. In consequences ofthis process, a finely divided precipitate (probably a mixture of silicaand calcium salts of the acid used) deposits on the surfaces and theinterstices of the fibers and is distributed throughout the mass offibers.

Either sized or unsized paper can be prepared according to thisinvention. If sized paper is desired, the size such as rosin size insoluble form is mixed with the pulp slurry and a precipitant for thesize is added together with or prior to the addition of the calciumsilicate. The precipitant is a compound of a trivalent or tetravalentmetal, and is preferably aluminum sulphate.

As has been stated above, sulfuric acid and sulphurous acid (as well astheir anhydrides S03 and S02 and the acidic salts thereof, such asaluminum sulphate, sodium bisulphite, ammonium sulphate, sodium sulphateand the like which hydrolyze to produce sulphuric or sulphurous acid,are particularly effective acidic materials for use according to thisinvention because of the high brightness and excellent opacityproperties of paper produced when such materials are used in conjunctionwith calcium silicate. Other acidic substances which form insolublealkaline earth metal salts and which are usable in preparing the productof this invention are phosphoric acid, phosphorous acid, oxalic acid,tartaric acid, and citric acid. Acid solutions which attack cellulosefiber should be avoided. Thus, where sulphuric acid is used, it isusually best to use relatively dilute acid.

The calcium silicate which has been found to yield superior results is afinely divided precipitated product which has the composition CaO(SiO2)mwhere x is 2 to 4 and which has an average ultimate particle size belowabout 0.1 micron.

A particularly suitable calcium silicate can be prepared by mixingflowing streams of calcium chloride and sodium silicate solutionstogether in a zone of high turbulence where almost instantaneous mixingcan be effected. While considerable variation in the concentrations ofthe reactant solutions is possible without affecting the particle sizeof the calcium silicate produced, a calcium chloride solution containing50 to 150 grams of salt per liter and a sodium silicate solutioncontaining 5 to 15 percent by weight of sodium silicate with a NazOISiOzratio varying from 1:2 to 1:4 are preferred. One way to produce therequired turbulence is to introduce the two streams closely togetherinto the central area of a centrifugal pump. Agitation of the mixture iseffected as the reactant streams are thrown outwardly by the pump rotor.To effect maX irnurn agitation, the amount of reacting solution suppliedto the pump is held below the capacity of the pump to deliver liquid,usually less than percent of the pump capacity, and preferably 65percent or less. Running the pump in this way causes the reacting massto remain in the zone of agitation for a longer period and insures theproduction of calcium silicate having the desired fineness. Calciumsilicate which precipitates is separated from the slurry in which it isformed, and is dried at a temperature ranging from about C. to 150 C.,generally at about C. The reaction whereby calcium silicate isprecipitated may be represented as follows:

CELCi2+NH20(SlOZ)z- Ca0(SiO2)+2NaCl where x is between 2 and 4.

Calcium silicate prepared by the above-described method has an averageultimate particle size of about 0.015 to 0.04 micron and a surface areaof about 50 to 100 square meters per gram. This calcium silicateproduces a high-quality product when used in the performance of thisinvention.

Naturally occurring forms of calcium silicate, such as wollastonite, andcalcium silicate prepared by methods other than that above indicated maybe used provided the product is readily reactive with the acids hereincontemplated. When precipitated calcium silicate is used, it frequentlyis preferred to dry the precipitate, for example, above about 100 C. butbelow the temperature at which the calcium silicate becomes crystalline(preferably below 800 C. and usually 100 to C.). The amount of calciumsilicate added to the acid slurry should be sufficient to establish asubstantial SiOz concentration in the fibers. Preferably the resultingproduct should contain at least 0.5 to 1 percent by weight of SiOz basedupon the dry weight of the paper pulp. Much larger amounts rang ing ashigh as 30 percent SiOz or even higher may be used. For most purposes,the products produced contain 0.5 to 10 percent, usually in the range of2 to 5 percent SiO2.

It will be understood that when calcium silicate is added to the acidicpulp slurry, acidic component is consumed and the pH of the slurryrises. If sufiicicnt calcium silicate is added, all of the acid isconsumed and, upon addition of further calcium silicate, the slurrybecomes alkaline. For most purposes, sufiicient acidic component shouldbe present to react with at least 0.5 to 1 pound of calcium silicate per100 pounds of dry pulp in the slurry and usually the amount of acidiccomponent is enough to react with substantially all of the calciumsilicate added. Excess calcium silicate may be added. However, thisexcess is rarely above that which will increase the pH of the pulpslurry above about 9 and usually the calcium silicate added does notexceed more than twice the amount which will be decomposed by the acid,i. e., usually 50 percent or more of the calcium silicate added isdecomposed.

Aluminum sulphate reacts with calcium silicate in the proportion ofabout one mole of aluminum sulphate to three moles of calcium silicate.

In performing the process according to the present invention, pulp isslurried in any suitable agitating vessel, and is then beaten. Theduration of heating is determined in accordance with the type of pulpbeing beaten and the type of paper product desired. Groundwond pulprequires longer beating than do the chemically treated pulps. Generally,long beating should be avoided as this results in the production of atransparent paper.

Size and coloring matter are added either at the beginning of or duringthe beating cycle. Rosin size in the form of a soluble alkali metalrosin soap is suitable. Coloring matter may be added if desired.

Toward the end of the beating cycle, say about 10 minutes to one-halfhour before the end, a precipitant for the size and an acidic substancewhich does not harm cellulose fibers are added to the beaten stock. Asabove stated, aluminum sulphate is especially useful since it serves forboth purposes. In a typical operation of the type contemplated, l to 10percent of aluminum sulphate hydrate, Al2(SOi)a'l8H2O, based on the dryweight of pulp may be used. If desired, aluminum sulphate may be addedtogether with another acidic substance to the beater. In that case, theamount of aluminum sulphate is sufiicient only to precipitate the size,and is about 1 to 3 percent of the dry weight of pulp, depending uponthe amount of size to be precipitated. Moreover, where other acidicmaterials are used, other precipitants may be used to precipitate thesize, e. g., salts of other trivalent or tetravalent metals such asTi(SO4)2.

After the size is precipitated, the calcium silicate or other alkalineearth metal silicate is added to the beaten acidic pulp slurry to form aprecipitate on the fiber surfaces. Severe dusting problems arise ifwater is agitated while silicate is being added; for this reason, it isdesirable to add all the silicate before the commencement of agitation,or to add it in beater bags. Broke may be included in the suspension ifdesired. The suspension of calcium or other alkaline earth metalsilicate, and broke if desired, in water is mixed with the slurry ofbeaten and acidified pulp with agitation, and the product of thisinvention is formed.

Following preparation of the pulp slurry, the pulp may be sheeted intopaper in any conventional manner. Water discarded from the sheetingoperation, commonly referred to as white water, has a pH of about 5 to6. The paper sheet is pressed and dried, and tub-sized if desired.Drying at a temperature under 100 C. for a long period of time isdesirable in order to obtain paper of high strength.

In certain tests described in the ensuing examples, pripcr producedaccording to this invention was tested for brightness, opacity, tearstrength, and bursting strength. The testing procedures were as follows:

Brightness-The percentage reflectance of ten sheets of paper wasmeasured on a Hunter multipurpose refiectometer with both green and bluelight.

Opacity.The contrast ratio method (TAPPI Standard T42Sm44) was used. Thereflectance of a single thickness Contrast ratio:

Retention= where:

Azpercent ash in air dry stock. Bzpereent ash in air dry paper.

=percent loss on ignition of lone dry filler. The factor 0.94 allows for6% fiber loss through the wire of the paper-making machine. The formulais taken from Sutermeister, Chemistry of Pulp and Paper Making, thirdedition, New York, Wiley, 1941, page 341. I The following examples areillustrative of the present invention:

EXAMPLE I A pulp beater was charged as follows:

Grams Unbleached sulfite pulp Bleached kraft pulp Hard white envelopecuttings 90 This charge was dispersed in 23 liters of water and theresulting slurry was beaten in a Niagara beater for about 80 minutes toapproximately 400 milliliters freeness, Canadian Standard (TAPPIStandard T227m50). One hundred milliliters of prepared rosin of about 5percent by weight size was then added. Thereafter, 9 grams ofpregelatinized starch was added and the pulp was circulated in thebeater long enough to insure complete mixing. Aluminum sulphate in theamount of 3 percent by weight, based on the dry weight of pulp, wasadded as a solution containing 12.92 grams of aluminum sulphateoctodecahydrate per liter. At this stage, the pulp consistency wasapproximately 1.1 percent. Stirring was continued for 2 minutes.Immediately thereafter, the amounts of calcium silicate having thecomposition CaO(SiO2)3.3 and titanium dioxide indicated in the tablewere added. The amounts are expressed as percentages of the dry weightof the pulp. Stirring of the mixture was continued for 10 minutes more,and the resulting stock was diluted to 8,000 milliliters, divided intoten SOD-milliliter portions, and sheeted into paper. The paper wastested for brightness, opacity, bursting strength, weight and thickness,and ash and moisture content, with the following results:

Table I Sample No.

Percent titanium dioxide byweight 1.0 0.5 None 1.5 None None Percentcalcium silicate by wei ht. 1.0 2.5 4.0 None 4.0 None Brightness:

Green light. 70.8 70.0 72.9 73.2 72.2 70.5 Blueliglit 61.2 02.2 615.363.1 till] 50.3 Contrast ratio (green light): Reflectance, black bncking56.9 57.0 57.0 57.9 57.3 13.0 Reflectance, white hackiug 79.0 80.1 81.180.0 70.7 70.0 Ratio 72.1 71.2 70.3 718 T1." tii'tti Contrast ratio(blur.- light):

Reflectance, black backing 53.7 53.9 54.5 54.9 54.7 49.0 Reflectance,white backint: 60.54 67.!) 69.3 08.6 68.1} 000 Ratio 70.2 75.3 74.5 75.970.0 70.0 Bursting strength, lb./

in) 32.0 34.6 33.0 33.6 85.0 35.0 Percent ash in dry paper 2. 63 2. H31.86 2. 84 1. 2i

Comparison of paper pigmented with the calcium silicate-aluminumsulphate product of this invention (samples 55 and 57) with paperpigmented with titanium dioxide (sample 56) shows that about the samebrightness and opacity is imparted by both pigments, and that bothpapers have about the same bursting strength. Hence, the relativelyexpensive titanium dioxide may be replaced with a reaction product ofthis invention at considerable saving in cost and without diminution ofphysical properties such as strength, brightness, or opacity. Titaniumdioxide may be used in the present process as in samples 53 and 54.

EXAMPLE II One thousand pounds of bleached kraft pulp and 1000 pounds ofbleached sulfite pulp were slurried in roughly 6000 gallons of water.The pulp was agitated after which it was beaten in a stainless steelJones beater at 3000 lb./ sq. in. pressure for 90 minutes. Twenty-fourpounds of rosin size in aqueous solution was added to the beater at thestart of the beating cycle, and 26 fluid ounces of Ultramarine blue,equivalent to 13 ounces of dry coloring matter, was added to the beaterone-half hour after the start of the beating cycle. Fifty-one gallons ofiron-free aluminum sulphate solution containing 3.5 pounds of AI2(SO4)3'181-120 per gallon was added to the beater onehalf hour before the endof the beating cycle. While beating was in progress, 120 pounds offinely divided calcium silicate prepared by the reaction of sodiumsilicate with excess calcium chloride as described above was slurriedwith 5 pounds of broke and about 1000 gallons of water and agitated forone hour. This slurry and the beaten pulp were mixed in a stock chest.The stock thus formed, which had a solids content of about 4 to 5percent, was pumped through a refiner and head box to a Fourdriniermachine. Free water containing small amounts of pigment and pulp wassucked through the wire and discarded, leaving a paper sheet which wasdried and tub-sized.

For comparison of paper pigmented according to the present inventionwith paper pigmented with titanium dioxide, a sample of the latter wasmade as follows:

One thousand pounds of bleached kraft pulp and 1000 pounds of bleachedsulfite pulp were slurried in roughly 6000 gallons of water. The pulpwas agitated after which it was beaten in a stainless steel Jones beaterat 3000 pounds per square inch roll pressure for 90 minutes. Twenty-fourpounds of rosin size in aqueous solution was added to the beater at thestart of the beating cycle. After beating had continued for one-halfhour, 50 pounds each of titanium dioxide and Huber X-43 clay were added.Twenty-four gallons of iron-free aluminum sulphate solution containing3.5 pounds of Al2(SO4)s' 18H20 per gallon was added to the beaterone-half hour before the end of the beating cycle. While heating was inprogress, 500 pounds of broke was slurried in about 1000 gallons ofwater and agitated for one hour. This slurry and the beaten pulp weremixed in a stock chest, and the resulting stock was pumped through arefiner and head box to a Fourdrinier machine, where the pulp wassheeted into paper as described above.

The two samples of paper were analyzed for ash composition, brightness,opacity (contrast ratio), pH, yellowing with age, bursting strength andtear strength, with the following results:

Table II Calcium 8111- Titanium Dieate Pigmented oxide Pigmented BeforeAfter Before After Tub Tub 'Iub Tub Size Size Size Size Weight, lb. perream (500 sheets,

22 x 34") 19. 4 20. 0 l9. 7 20.0 Thickness, mils per sheet 3. 8 4. 1 3.83.9 Percent ash in oven dry paper 4. 42 4. 41 4. 55 4. 21 Analysis ofash:

Percent, S10, 55. 6 56. 3 20. l 19. 7

Percent R10; (including T10,) 36. 4 36. 2 74. l 74. 5 Brightness percentreflectance]:

Blue llg t, felt side 82. 7 82. 6 84. 5 B3. 1

Blue light, wire side 82.3 82. 9 84. 6 83.3

Green light, felt side. 83. l 83. 2 83. 9 S3. 4

Green light, wire side 82. 6 84. 0 84. 2 83. 0 Contrast ratio:

Blue light H 85. 8 84. 2 88. 3 85. 3

Green light 82. 0 80. 0 84. 2 80. 9

'White light 80. 6 79. 5 83. 6 80. 5 pH (TAPPI Standard T435-M42).-. 6.56. 6 ti 7 6. 7 Fadeometer Test:

Brightness before test 82. 7 82. 5 84. 5 83.0

Brightness after 24 hours exposure at F 78.3 77. 4 79. 4 78. 6

Change 4. 4 5.1 5. l 4. 4 Bursting Strength (Mullen)- 22. 7 32. 7 2'4. 532. 1 Tear Strength (Elmendorf):

Machine direction 55 50 48 43 Cross direction 57 51 52 49 The abovetable shows that a paper having high brightness and opacity andsatisfactory strength can be prepared by the present invention and thatthe paper thus prepared is essentially equivalent in brightness andopacity to paper pigmented with titanium dioxide.

EXAMPLE III A beater was charged with 600 pounds of rag fiber, 600pounds of bleached kraft pulp, 1000 pounds of bleached sulfite pulp, 100pounds of titanium dioxide, 32 pounds of rosin size in aqueous solution,and about 6000 gallons of water. The charge was beaten for about an hourand a half, and 16 fluid ounces of blue dye solution, and 45 gallons ofiron-free aluminum sulphate solution containing 3.5 pounds ofAl2(SO4)a-l8H2O per gallon were dumped into the beater 15 minutes beforethe end of the beating cycle. While beating continued, 200 pounds offinely-divided calcium silicate prepared by the reaction of sodiumsilicate with excess calcium chloride was placed in a hydrapulpertogether with 1200 pounds of broke consisting of scrap paper pigmentedwith titanium dioxide and about 2000 gallons of water. The hydrapulperwas then started, and agitation continued for one hour. The hydrapulpedslurry was mixed with two beaterloads of pulp in a stock chest. Thestock was then pumped through a refiner and head box to a Fourdriniermachine, where paper sheet was formed. The paper sheet was dried and tubsized.

For comparison, a paper sheet containing no calcium silicate was made.The procedure described in the previous paragraph was followed, exceptthat pounds of titanium dioxide and only 24 gallons of aluminum sulphatewere used. All the pigment was admixed with the Results of tub-sizedplug and rag fiber 1n the beater. samples of the two papers are asfollows:

Table III Calcium Titanium Silicate Dioxide Pigmented Pigment OnlyWeight, lb. per ream (500 sheets, 22" x 34") 19. 7 20. 6 Thickness, milsper sheet i. 4. 1 4. 2 Brightness (percent reflectance, blue light):

Felt side 85. 0 83. 3 Wire side 84. 3 84. 5 Contrast ratio (green light)86. l 85. 7 )H (TAPPI Standard 1135-. 6. 5 6. 6 ursting Strength 30.232. 0 Tear Strength:

Machine direction 50 50 Cross direction 55 52 Comparison of theproperties of paper containing calcium silicate with paper pigmentedonly with titanium dioxide shows no material difference between the two.About 4 to pounds of calcium silicate per 100 pounds of dry pulp,together with sufiicient acidic material such as aluminum sulphate toneutralize at least one-half of the lime content of the silicate,produces paper with similar optical properties to that produced bypigmentation with about 1 /2 to 3 percent of titanium dioxide based onthe dry weight of pulp.

EXAMPLE IV Three hundred and eighty-five grams of kraft pulp wasslurried in 23 liters of water, soaked two hours, and beaten for threehours in a Niagara beater at 6500 grams bar load. Fourteen hundred gramsof pulp slurry was diluted to 5900 grams, and 16.8 cc. of 5 percentaluminum sulphate solution, which corresponds to 4 percent by weight ofaluminum sulphate based on the dry weight of pulp, was added. The pulpslurry was stirred five minutes.

A slurry containing 1.05 grams of dried precipitated calcium silicate.CaO(SiO2)3.s, or about 5 percent by weight based on the dry weight ofpulp, in 400 milliliters of water was prepared. This slurry was added tothe pulp slurry and the mixture was stirred for five minutes.

This procedure was repeated in successive runs in which part of thealuminum sulphate was replaced by a l percent sulfuric acid solution.The amounts of each reagent in each run was as follows:

cc. of 5% Al: cc. 011% (SO4)3. 181110 H2504 16. 8 None Table IV I Run 1Run 2 Run 3 Run 4 Brightness (percent blue reflectance). 73.9 75. 7 75.3 75. 6 Contrast ratio (green. light) 66. 9 67. 2 6T. 3 66. ll Burstingstrengtl1..... .1 2T. 2 27. 2 26. 8 26. 4

Replacement of part of the aluminum sulphate with sulfuric acid resultsin a paper of improved brightness, about the same opacity, and slightlydecreased bursting strength compared to paper produced using aluminumsulphate. Since sulfuric acid is much cheaper than aluminum sulphate,these results show the further advantage which accrues by use ofsulfuric acid as the acidic substance and using aluminum sulphate onlyin suflicient quantities to precipitate the size. Other sulphur-oxygencontaining acids, notably sulphurous acid including its anhydride S02,may be used with the same results.

The above description has been directed primarily to embodiments of theinvention wherein the calcium silicate is prepared by continuous mixingof flowing streams of sodium silicate and calcium chloride and thecalcium silicate has the composition CaO(SiO2)1 where x is about 2 to 4.However, calcium silicate of somewhat lower surface area produced by abatch process in which the sodium silicate is added to a pool of calciumchloride solution or vice versa also may be used. Moreover, other waterinsoluble calcium silicates including mixed silicates of calcium andother metals may be used, particularly when the silicates have anaverage ultimate particle size below about 1 micron, preferably below0.1 micron. Such silicates include calcium aluminum silicates, calciumsodium aluminum silicates, calcium potassium, aluminum silicates,calcium zinc silicates, and like silicates in which the mole ratio oftotal Slog to the total CaO in the product is not substantially greaterthan about 10. A typical mixed silicate of this type which recently hasbecome available has the following composition:

Percent Ignition loss at 1000 C 16.7 Loss in drying at C 5.4 SiO2 41.1R201; (almost entirely AlzOs) 10.88 F6203 0.24 CaO 11.82 MgO 0.38 NazO10.04 Chloride 0.04 S03 3.8 C02 6.6

Some portion of the CaO content of this product appears to be present ascalcium carbonate.

Magnesium silicate, strontium silicate, or barium silicate may be usedin place of calcium silicate in any of the foregoing examples.

Other woven, felted, or slurried fibrous materials, such as cotton,silk, rayon, linen, nylon, fibers of polymeric acrylonitrile andacrylonitrile copolymers, dacron, hemp, sisal, etc. may be treated inthe same manner by impregnating with an acidic material of the typedescribed above and reacting the product with an alkaline earth metalsilicate.

This application is a continuation of applicants prior applicationSerial No. 352,295, filed April 30, 1953, now abandoned.

While the present invention has been described with particular referenceto illustrative examples of several embodiments hereof, the invention islimited only by the scope of the appended claims.

What is claimed:

1. A method of preparing paper which comprises forming an aqueous slurryof cellulosic paper pulp and an acid substance which forms a Waterinsoluble alkaline earth metal salt, reacting with said slurry finelydivided alkaline earth metal silicate and producing paper from theresulting slurry, the amount of said acidic substance being sufiicientto react with at least half the alkaline earth metal silicate.

2. A method of preparing paper which comprises forming an aqueous slurryof cellulosic paper pulp and an acid substance which forms a waterinsoluble calcium salt, reacting with said slurry finely divided calciumsilicate and producing paper from the resulting slurry, the amount ofsaid acidic substance being sutficient to react with at least half thecalcium silicate.

3. A method of preparing paper which comprises forming an aqueous slurryof cellulosic paper pulp and aluminum sulphate, reacting with saidslurry finely divided calcium silicate and producing paper from theresulting slurry, the amount of said aluminum sulphate being sufficientto react with at least half the calcium silicate.

4. A method of preparing paper which comprises forming an aqueous slurryof cellulosic paper pulp and aluminum sulphate, reacting with saidslurry finely divided calcium silicate having at least two routes ofSiO2 per mole of CaO and producing paper from the resulting slurry, theamount of said aluminum sulphate being sufficient to react with asubstantial portion of the calcium silicate.

5. A method of preparing paper which comprises forming an aqueous slurryof cellulosic paper pulp and aluminum sulphate, reacting with saidslurry finely divided calcium silicate prepared by reaction of sodiumsilicate and a calcium salt and producing paper from the resultirigslurry, the amount of said aluminum sulphate being sulficient to reactwith a substantial portion of the calcium silicate.

6. The method of claim 5 wherein the calcium salt is calcium chloride.

7. A method of preparing paper which comprises forming an aqueous slurryof cellulosic paper pulp and aluminum sulphate, adding to and reactingwith said slurry finely divided calcium silicate having from two to fourmoles of SiOz per mole of CaO and prepared by reaction of sodiumsilicate and calcium chloride, the amount of aluminum sulphate beingsufiicient to react with a substantial portion of the calcium silicateand producing paper from the resulting slurry, the amount of said addedcalcium silicate being suflicient to establish in the paper an Si02concentration of 0.5 to 30 percent by weight.

8. A method of preparing paper which comprises forming an aqueous slurryof cellulosic paper pulp and aluminum sulphate, reacting with saidslurry finely divided alkaline earth metal silicate and producing paperfrom References Cited in the file of this patent UNITED STATES PATENTS228,328 Ehrhardt June 1, 1880 1,345,317 Clapp June 29, 1920 2,237,374Smith Apr. 8, i941 2,599,094 Craig June 3, l952

1. A METHOD OF PREPARING PAPER WHICH COMPRISES FORMING AN AQUEOUS SLURRYOF CELLULOSIC PAPER PULP AND AN ACID SUBSTANCE WHICH FORMS A WATERINSOLUBLE ALKALINE EARTH METAL SALT, REACTING WITH SAID SLURRY FINELYDIVIDED ALKALINE EARTH METAL SILICATE AND PRODUCING PAPER FROM THERESULTING SLURRY, THE AMOUNT OF SAID ACIDIC SUBSTANCE BEING SUFFICIENTTO REACT WITH AT LEAST HALF THE ALKALINE EARTH METAL SILICATE.